This invention relates to stepper motors of the rotary or linear type.
A stepper motor is an electromagnetic incremental actuator. A stepper motor of the variable reluctance or permanent magnet type includes a magnetic stator structure and a magnetic moving structure which comprises a rotor in the rotary motor case and a slider in the linear motor case. Either the moving structure or the stator structure has windings associated with various pole positions which are sequentially and selectively energized to produce incremental motion of the moving structure.
The stationary positions of linear and rotary stepper motors after each step, hereinafter referred to as the "step positions," are inherent in the magnet structure. In other words, the moving structure will move to a predetermined, stable stepped position is response to the energization of one or more windings.
In many applications, this incremental motion as provided by stepper motors is particularly desirable. One such application is found in printers of the type disclosed in copending application Ser. No. 809,646 filed June 24, 1977 wherein a linear stepper motor is utilized to advance a movable print point in a serial impact printer. The magnetically inherent step positions of a rotary stepper motor may be utilized to position a daisy character element in a serial impact printer as disclosed in copending application Ser. No. 809,923 filed June 24, 1977.
However, the magnetically inherent step positions of a linear or rotary stepper motor may be insufficient or inadequate in many applications including serial impact printers. For example, the magnetically inherent step positions may not provide a sufficient number of steps in a printer where very small steps are required as necessitated by certain print characters or certain spacing between print characters. In addition, the magnetically inherent step positions may be improperly located.
In this connection, it will be understood that a very high degree of precision is required of a printer linear stepper motor associated with a carriage as well as the rotary stepper motor associated with the print element. However, such precision may be difficult to achieve in the magnetic structure although the discrete steps of the stepper motor still provide distinct positioning control advantages in approaching the step position. In other words, the stepper motor affords control advantages in coarse positioning but may be inadequate for fine positioning.
Heretofore, feedback or closed-loop control of stepper motors has been utilized to control the selective energization of the motor winding. See Theory and Applications of Step Motors, Kuo, West Publishing Company 1974, pp. 252-272 and 279. However, the closed loop or feedback control has not been utilized to supplement or modify the inherent step positions but merely to control the motor in reaching those inherent step positions.
In U.S. Pat. No. 3,906,326, a DC motor is stopped at predetermined positions using optical feedback to position the motor However, the motor is not of the stepper type and there is therefore no effort to supplement or change the inherent step positions of such a motor since the DC motor disclosed has no such step positions.